Self-oriented bundles of carbon nanotubes and method of making same

ABSTRACT

A field emission device having bundles of aligned parallel carbon nanotubes on a substrate. The carbon nanotubes are oriented perpendicular to the substrate. The carbon nanotube bundles may be up to 300 microns tall, for example. The bundles of carbon nanotubes extend only from regions of the substrate patterned with a catalyst material. Preferably, the catalyst material is iron oxide. The substrate is preferably porous silicon, as this produces the highest quality, most well-aligned nanotubes. Smooth, nonporous silicon or quartz can also be used as the substrate. The method of the invention starts with forming a porous layer on a silicon substrate by electrochemical etching. Then, a thin layer of iron is deposited on the porous layer in patterned regions. The iron is then oxidized into iron oxide, and then the substrate is exposed to ethylene gas at elevated temperature. The iron oxide catalyzes the formation of bundles of aligned parallel carbon nanotubes which grow perpendicular to the substrate surface. The height of the nanotube bundles above the substrate is determined by the duration of the catalysis step. The nanotube bundles only grow from the patterned regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of patent application 09/191,728, nowU.S. Pat. No. 6,232,706, filed Nov. 12, 1998, which is hereinincorporated by reference, to which priority is claimed under 35 U.S.C.§120.

FIELD OF THE INVENTION

This invention relates generally to carbon nanotubes. The presentinvention also relates to methods for making bundles of aligned carbonnanotubes. The present invention is also related to carbon nanotubefield emission devices such as used in flat panel displays.

BACKGROUND

Field emission devices have potential applications in flat paneldisplays. Field emitters used in flat panel displays must be stable,long lasting and should have a relatively uniform emission over thesurface of the display.

Carbon nanotubes are very small tube-shaped molecules having thestructure of a graphite molecule rolled into a tube. Carbon nanotubesare electrically conductive along their length, chemically stable, andcan have very small diameters (much less than 100 nanometers) and largeaspect ratios (length/diameter). Due to these properties and otherproperties, it has been suggested that carbon nanotubes can be used asfield emission devices.

However, it has been unclear how to realize a field emission deviceexploiting carbon nanotubes. They are difficult to work with in bulk,and, on a microscopic level, often form an impossibly tangled messresembling a hairball. To produce a useful field emission device forflat panel displays, the carbon nanotubes should be patterned intoindividual field emitters. A problem with present methods of makingcarbon nanotube field emitters is that it is not clear how to patternthe carbon nanotube to provide arrays of emitters.

U.S. Pat. No. 5,773,921 to Keesman et al. discloses a field emissiondevice which employs sharp-edged graphite wafers. Carbon nanotubes aredisposed on the sharp edge of the graphite wafers and help to increasefield emission. The carbon nanotubes are disposed on the graphite wafersby sputtering a nearby graphite target. The carbon nanotubes are notaligned in any way.

U.S. Pat. No. 4,272,699 to Faubel et al. discloses an ion source whichuses carbon fiber field emitters. The carbon fibers are bundled togetherand an electric field is applied to the bundle. The carbon fibers areheld by a macroscopic mechanical device that holds the fibers parallel.Such a mechanical device cannot be used with carbon nanotubes, which areorders of magnitude smaller than the carbon fibers used by Faubel et al.

U.S. Pat. No. 5,773,834 to Yamamoto et al. describes a method of makingcarbon nanotubes on the surface of a carbon-containing substrate by ionbombardment. The carbon nanotubes produced can be used as fieldemitters. The carbon nanotubes produced according to Yamamoto are notaligned, and in particular, are not aligned perpendicular to thesubstrate. Also, Yamamoto does not disclose how to pattern the substrateto provide individual field emitters.

Yet another problem with present methods for making carbon nanotubefield emitters is that scale-up to large wafers is difficult or notpossible.

There exists a need in the art for a method of producing aligned carbonnanotubes. In particular, such aligned carbon nanotubes can be used assuperior field emission devices.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to providing:

1) a method of making aligned bundles of carbon nanotubes; and

2) a field emission device using aligned bundles of carbon nanotubes asfield emitters;

3) a field emission device that can have arrays of emitters;

4) a method of making field emitters that can be scaled to largesubstrates.

These and other objects and advantages will be apparent upon reading thefollowing description and accompanying drawings.

These and other advantages will be apparent upon reading the followingdescription and accompanying drawings.

SUMMARY

In one example embodiment of the present invention, a field emissiondevice includes a refractory substrate composed of silicon or quartz, acatalyst material on top of the substrate, and a bundle of alignedparallel carbon nanotubes extending from the catalyst material in adirection perpendicular to the substrate.

In one example embodiment of the present invention, the substrate has atop layer of porous silicon, with the catalyst material disposed on theporous layer. Particularly, the porous layer can be composed of an uppernanoporous layer with small pores on top of a macroscopic porous layerwith larger pores. The catalyst material is iron oxide.

The substrate may also have a smooth, nonporous surface, or a roughsurface.

The carbon nanotube bundles may be within 10-22 nanometers in diameterand may be up to 300 microns tall. Also, the carbon nanotubes bemulti-walled.

In another example embodiment of the present invention, the catalystmaterial is confined to a patterned region. This results in the bundleextending from the patterned region. The bundle has the same footprintsize and shape as the patterned region of the catalyst material.

The carbon nanotube bundles may have a flat top, or a bowl-shaped top.

The present invention also includes a method of making bundles ofaligned carbon nanotubes on a substrate of silicon or quartz. The methodincludes the steps of depositing a catalyst material on a top surface ofthe substrate, and then exposing the substrate to a carbon containinggas.

In one implementation, the substrate is a silicon substrate with aporous top surface. The top surface may be made porous byelectrochemical etching. In another example embodiment of the presentinvention, the catalyst material is iron oxide. The iron oxide may bedeposited by depositing a thin film of iron, and then oxidizing the ironfilm. The iron film may be oxidized by exposing it to oxygen at elevatedtemperature. In one particular instance, the original iron film is bout5 nanometers thick.

The carbon containing gas may be ethylene.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and detailed description that follow moreparticularly exemplify these embodiments.

DESCRIPTION OF THE FIGURES

The invention may be more completely understood in consideration of thedetailed description of various embodiments of the invention thatfollows in connection with the accompanying drawings, in which:

FIG. 1 shows a field emission device, according to an example embodimentof the present invention;

FIG. 2 shows a graph of field emission characteristics of a fieldemission device, according to another example embodiment of the presentinvention;

FIG. 3 illustrates a method for making bundles of aligned carbonnanotubes on a porous silicon substrate, according to another exampleembodiment of the present invention;

FIG. 4 illustrates a method for making bundles of aligned carbonnanotubes on a smooth, nonporous silicon substrate, according to anotherexample embodiment of the present invention; and

FIG. 5 shows a field emission device with many nanotube bundles, eachbundle providing field emission to illuminate a single pixel in a flatpanel display, according to another example embodiment of the presentinvention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is believed to be applicable to a variety ofdifferent types of devices, and the invention has been found to beparticularly suited for field emission devices and other devicesemploying carbon nanotubes. While the present invention is notnecessarily limited to such applications, various aspects of theinvention may be appreciated through a discussion of various examplesusing this context.

FIG. 1 shows a field emission device 20 according to an exampleembodiment of the present invention. The device has a substrate 22 witha porous top layer 24. The substrate 22 and top layer 24 are made ofsilicon, although other substrate materials can also be used. Disposedon the porous top layer 24 are patterns of catalyst material 26. In oneinstance, the catalyst material 26 is a thin film of iron oxide.Extending from the catalyst material patterns 26 are carbon nanotubebundles 28, which are perpendicular to the substrate 22. The carbonnanotubes comprising the bundles are parallel and perpendicular to thesubstrate 22. The carbon nanotubes comprising the bundles are typicallyabout 10-22 nanometers in diameter.

The bundles 28 may be about 10-250 microns wide 30, and up to orexceeding 300 microns in height 32. The nanotube bundles have the samewidth 30 as the catalyst material patterns 26. More generally, thebundles 28 have the same ‘footprint’ size and shape as the catalystmaterial patterns 26. The nanotube bundles 28 can have a flat top 34, orcan have a bowl-shaped top 36, depending on the process parameters usedto make the nanotube bundles. There are no individual stray nanotubesextending away from the blocks.

The nanotube bundles 28 have sharp edges 38 and corners that serve asfield emission regions. Since the substrate can be conductive (e.g.doped silicon) and the nanotubes are conductive along their length(parallel with height 32), electrical connections to the bundles 28 aremade simply by connecting to the substrate 22.

FIG. 2 shows field emission characteristics for four nanotube bundles ona n⁺ type silicon substrate. The bundles each have a footprint of 250microns×250 microns and are 160 microns tall. The anode is an aluminumcoated silicon wafer 200 microns above the substrate. The data is takenin a vacuum chamber at 3×10⁻⁷ Torr base pressure. Note that the electricfield is computed by using the applied voltage divided by the distancefrom the tops of the bundles to the anode (40 microns). The currentdensity reaches 10 mA/cm² at an electric field strength of 5volts/micron. After continuous emission for a week at 2 mA/cm² thebundles show no indication of damage under a scanning electronmicroscope.

The present invention includes methods of making the carbon nanotubebundles attached to substrates. FIG. 3 illustrates a preferred methodfor making the nanotube bundles. First, in step A, a highly P-doped n⁺type silicon substrate 22 (100 top surface, resistivity 0.008-0.018Ohm-cm) is electrochemically etched in 1:1 HF (49% in water) ethanolwith an anodization current density of 10 mA/cm² (typical etching timeis 5 minutes). This forms a thin nanoporous layer 42 (pore size˜3nanometers) on top of a macroporous layer 44 (pore size˜100 nanometers).Layers 42, 44 are generally referred to as a porous layer 24 in FIG. 1.Next, in step B, the porous layer 42 is patterned with a 5 nanometerthick iron film 46 by e-beam evaporation through a shadowmask 48. Afterdeposition of iron, the substrate is annealed in air at 300° C.overnight. This annealing step oxidizes the surface of the silicon aswell as the iron, converting the iron patterns 46 into catalyticallyactive iron oxide patterns 26. The resulting silicon dioxide layerformed on the porous silicon prevents the porous structure of layers 42,44 from collapsing during the following high temperature chemical vapordeposition (CVD) step. Next, in step C, the substrate 22 is placed in a2-inch tube reactor 49 housed in a tube furnace. The furnace is heatedto 700° C. in flowing argon. Then, at 700° C., the argon supply isturned off, and ethylene 50 is flown through the tube reactor at a rateof 1000 sccm/min for 15-60 minutes. The boat for the substrates issealed at one end, and the sealed end is placed downstream in thefurnace. While ethylene 50 is flowing, the iron oxide patterns 26catalyze the growth of carbon nanotube bundles 28 which growperpendicular to the substrate 22. The width of the bundles 28 is thesame as the width of the iron oxide patterns 26.

The above method for making nanotube bundles produces mainly flat-topbundles, although sometimes bowl-shaped bundles are also produced.Bowl-shaped bundles are produced when nanotubes in the middle of abundle grow slower than the nanotubes at the outer edges of a bundle.

The height of the bundles 28 is determined by the duration that thesubstrates are exposed to ethylene at high temperature. In a particularexperiment conducted by applicants, reaction times of 5, 15, 30, and 60minutes produced bundles 31, 98, 163, and 240 microns tall,respectively. The growth rate is observed to be linear initially andthen falls off at longer reaction times. As the aspect ratio(height:width) of the bundles approaches 5:1, some bundles may becometilted, but refuse to fall down.

The bundles can have footprint sizes as small as 2 microns on a side andbe 15 microns tall, and still remain standing on the substrate.

Since the carbon nanotubes grow only from those regions that havedeposited iron oxide, patterning the iron oxide results in carbonnanotubes growing only from those regions with iron oxide. This providesaccurate control of the size, shape, and distribution of the bundles onthe substrate surface. As a particular advantage, patterning of thecatalyst provides the ability to fabricate arrays of individual fieldemitters. Each bundle can provide field emission for a single pixel in aflat panel display. Each bundle can be individually controlled byconnecting patterned metallization lines to the bundles. Further, thearrays can be formed on large substrates. The size of the substrate isonly limited by the size of the tube furnace. The substrate can beseveral inches across, for example. Many field emitter arrays have beenfabricated on 3 cm×3 cm silicon substrates.

The method of the present invention produces carbon nanotube bundleshaving a high density. The nanotubes are held together by Van der Waalsinteractions. The nanotubes grown according to the present method are inthe size range of about 16±6 nanometers and are aligned parallel in adirection perpendicular to the substrate 22. The carbon nanotubes arealigned perpendicular to the substrate regardless of the orientation ofthe substrate in the furnace. The carbon nanotubes are generallymulti-walled.

The porous layer acts as an excellent catalyst support. During the 300°C. annealing step, iron oxide particles form with a narrow sizedistribution due to their strong interactions with the porous layer. Thestrong interactions also prevent the iron oxide particles from sinteringat elevated temperatures.

It has been confirmed by the applicants that the nanotube bundles growin ‘base growth’ mode. This has been established by physically removingnanotube bundles from the substrate and observing that the substratesremain capable of growing carbon nanotubes (i.e. the iron oxide catalystpatterns remain on the substrate).

The present method of growing aligned nanotubes is very different fromprior art approaches of growing aligned nanotubes. In prior artapproaches, carbon nanotube alignment is provided by confining thegrowth of nanotubes to channels in porous silica or the channels ofalumina membranes. In the present method, the carbon nanotubes are selfaligned, without being confined to a channel, pore, or hole. The carbonnanotubes of the present method spontaneously align themselves in freespace.

It is important to note that the holes and pores in the porous layers42, 44 are not aligned or oriented in any way. For example, they are notnecessarily aligned perpendicular to the substrate 22. The holes andpores of the porous layer 24 generally have random orientations.

The present invention also includes a method of producing aligned carbonnanotubes on a nonporous smooth silicon substrate. The present methodfor growing aligned carbon nanotubes on nonporous silicon substrates islargely the same as the method for porous substrates. FIG. 4 illustratesa first step in the present method for producing aligned carbon nanotubebundles on a smooth nonporous silicon substrate 40. A thin film (e.g. 5nanometers thick) of iron 46 is deposited through a shadowmask 48 usingthe same technique as in step B of FIG. 3. The nonporous substrate 40has a native oxide layer which is left intact prior to the irondeposition. The 300° C. annealing and 700° C. CVD nanotube growth stepsfor nonporous substrates 40 are the same as in the method describedabove for porous substrates. The CVD process forms carbon nanotubebundles which extend from iron oxide patterns perpendicularly to thenonporous substrate 40.

The overall features of carbon nanotubes grown on nonporous silicon aresimilar to the features of nanotubes grown on porous silicon. However,in contrast to porous silicon substrates, nanotube bundles with aspectratios greater than 5 tend to fall onto the substrate 40. Nanotubebundles grown on smooth nonporous substrates are not as strongly boundto the substrate. Also, carbon nanotube bundles grown on nonporoussilicon substrates tend to have higher defect densities, and tend to beless well aligned than bundles grown on porous silicon. Further, carbonnanotubes grow about 50% faster on porous silicon compared to nonporoussilicon. For these reasons, the use of porous silicon substrates ishighly preferred.

FIG. 5 shows a field emission device that can be used in a flat paneldisplay. The device has many carbon nanotube bundles 28. Each bundle 28can provide field emission for a single pixel in a flat panel display.

It is also noted that the iron catalyst does not necessarily need to bedeposited using physical vapor deposition techniques, although this ispreferred. For example, catalyst materials can be deposited as ironsalts dissolved in a carrier solvent. The solvent is then deposited onthe substrate and allowed to dry, leaving the iron salt. The iron saltmay then need to be ‘activated’ by exposing it to high temperature sothat it decomposes into active catalyst material.

It is also noted that the substrate does not necessarily need to besilicon, although silicon, and particularly, porous silicon, ispreferred. The substrate can also be quartz. In the present application,silicon, porous silicon, and quartz are understood to be refractorymaterials.

The present method for making bundles of aligned carbon nanotubes mayalso work on substrates such as ceramics, alumina, sapphire, and silica,for example. The substrate must be able to tolerate the hightemperatures (about 700° C.) used in the CVD process without meting ordisintegrating. For best results, the substrate should have a rough andcomplex surface topology.

It is also noted that the substrate can have a rough texture which isneither smooth nor ‘porous’. Generally, however, extremely complexsubstrate surface topologies are preferred because they produce fastgrowing nanotubes with few defects that are strongly bound to thesubstrate.

It will be clear to one skilled in the art that the above exampleembodiments may be altered in many ways without departing from the scopeof the invention. Accordingly, such changes and implementations do notdepart from the spirit and scope of the present invention, which is setforth in the following claims.

1. A field emission device comprising: a) a substrate; b) a catalystmaterial on a porous surface of the substrate; c) one or more bundles ofparallel carbon nanotubes extending from the catalyst material in adirection perpendicular to the substrate; wherein the substratecomprises a material selected from the group consisting of ceramics,alumina, sapphire, and silica.
 2. A method of making bundles of alignedcarbon nanotubes on a porous surface of a substrate, the methodcomprising the steps of: a) e-beam evaporating a catalyst materialthrough a shadowmask on the porous surface such that one or morepatterned regions are produced; and b) exposing the catalyst material toa carbon containing gas at an elevated temperature such that one or morebundles of parallel carbon nanotubes grow from the one or more patternedregions in a direction substantially perpendicular to the substrate. 3.A field emission device comprising: a catalyst material deposited in anordered arrangement on a porous surface of a substrate; and at least onecarbon nanotube extending from the catalyst material in a directionsubstantially perpendicular to the substrate.
 4. The field emissiondevice of claim 3, wherein the at least one carbon nanotube includes aplurality of substantially parallel carbon nanotubes extending from thecatalyst material in a direction substantially perpendicular to thesubstrate.
 5. The field emission device of claim 3, wherein the at leastone carbon nanotube includes a bundle of substantially parallel carbonnanotubes extending from the catalyst material in a directionsubstantially perpendicular to the substrate.
 6. The field emissiondevice of claim 3, wherein the substrate comprises a material selectedfrom the group consisting of ceramics, alumina, sapphire, and silica. 7.The field emission device of claim 3, wherein the substrate includes apatterned region.
 8. The field emission device of claim 3, wherein theat least one carbon nanotube has a diameter of less than about 22nanometers.
 9. The field emission device of claim 3, wherein the atleast one carbon nanotube is multi-walled.
 10. The field emission deviceof claim 3, wherein the at least one carbon nanotube comprises at leastone graphite molecule rolled into a tube.
 11. The field emission deviceof claim 3, wherein growing at least one carbon nanotube includesforming a field emitter comprising the at least one carbon nanotube. 12.The field emission device of claim 3, wherein the at least one carbonnanotube has an edge portion that is a field emission region.
 13. Thefield emission device of claim 3, wherein the porous surface comprisesrandomly-oriented pores.
 14. A method of making carbon nanotubes on aporous surface of a substrate, the method comprising the steps of:forming at least one patterned catalyst region on the porous surface;and exposing the catalyst to a carbon containing gas at an elevatedtemperature and growing at least one carbon nanotube from the at leastone patterned region in a direction substantially perpendicular to thesubstrate.
 15. The method of claim 11, wherein growing at least onecarbon nanotube includes growing a bundle of aligned carbon nanotubes.16. The method of claim 14, wherein growing at least one carbon nanotubeincludes growing a plurality of substantially parallel carbon nanotubesextending from the catalyst in a direction substantially perpendicularto the substrate.
 17. The method of claim 14, wherein growing at leastone carbon nanotube includes forming a field emitter portion of a fieldemission device.
 18. The method of claim 14, wherein forming at leastone patterned catalyst region on the porous surface includes e-beamevaporating a catalyst material through a shadowmask on the poroussurface and producing the at least one patterned catalyst region.
 19. Afield emission device comprising: a catalyst material deposited in anordered arrangement on a porous substrate; and at least one carbonnanotube extending from the catalyst material in a directionsubstantially perpendicular to the substrate.
 20. The field emissiondevice of claim 19, wherein the nanotube includes a carbon moleculehaving a tubular form.